If you were to conduct a survey, most people would probably have little difficulty expressing some fundamental differences. After all, we learn to tell boys apart from girls in early childhood.

Answers in the survey might revolve largely around differences between the sexes in anatomy (including genitalia of course), or might even extend to sex-specific or sex-biased roles in reproduction (which sex gives birth, lactates, is typically the primary carer, and so forth).

alessandro_eye

While arguably adequate for defining the sexes in humans, these traits are largely inadequate for use in a broader definition of what is male and what is female across the animal kingdom.

Most birds do not have obvious genitalia. How many people could pick the sex of a sea urchin (pictured left) or a clam simply by looking at it?

In many animal species, males do not have anything remotely similar to an inseminating organ. There are also many examples of animals where the males and females do not subscribe to what we perceive as traditional sex roles – such as some fish and bird species, in which only the male looks after the eggs. In the case of seahorses, the males actually physically carry and incubate the offspring.

Chromosomes

So, if not based on anatomical features, can sex be traced to the level of chromosomes? The XY sex-determination system is well-known because it occurs in humans, many vertebrates and some insects.

In this system males have two kinds of sex chromosomes (XY), while females have two copies of the same sex chromosome (XX). But in birds, and a number of reptiles and butterflies, the reverse patterns is the norm (ZW for females, ZZ for males).

Sexual chromosomes are also paramount in controlling hormones that unleash profound developmental changes that are in many cases easily identifiable. Without doubt, we all use a range of such hormone-induced indicators to determine who is a male and who is a female on a daily basis, at least when it comes to humans (e.g. males have more facial hair than females).

Hence, at first glance, sex chromosomes may appear to be the way to go in defining sex. Look a bit deeper, though, and this approach is also far from watertight.

Charles & Clint

In many reptiles, sex determination comes via the temperature experienced by the embryos. And then there are species in which individuals spend one part of their life as one sex and another part as the other.

Some even live their lives as males and females simultaneously (so called hermaphrodites), such as the land snails (pictured right) that wreak damage to our back-garden vegetable crops each spring.

Matters of size

So is there any character we can single out that would adequately define the sexes? Well, yes, there is.

The answer relates to the size of cells that will fuse together to produce the next generation (the sex cells or gametes). The individual that produces the relatively smaller gamete is universally called “male” and the individual that produces the relatively larger gamete “female”.

Following from this, by definition, the evolutionary origin of males and females lies in the transition from isogamy (when the gametes in all individuals are all of equal size) to anisogamy (sexual reproduction involving sex cells of different sizes).

But, but …

You might be hearing a little voice nagging at the back of your head, pointing out that males and females are different in so many ways beyond the size of their gametes - doesn’t this matter at all?

Of course it does – but we would argue one thing leads to another.

Males produce sperm (the name for the smaller sex cells) and females produce ova/eggs (the name for the larger sex cells), but these are just names that we have chosen to give to the sexes and their sex cells - we could just as well call them something entirely different.

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So the crucial point is not the terminology. What matters is that anisogamy is at the heart of many of the other differences arising between the sexes, including behavioural differences and sexual preferences.

We can see why that’s the case by dissecting the following points:

1) Numbers. Imagine you have 1kg of flour to make muffins. If you want large muffins you can only cook a few of them, but if the muffins are really small you can make plenty more. So, the first implication of sexual asymmetry in the size of the gametes is that there is also asymmetry in the number of gametes produced. Females generally produce a few large gametes, while males typically produce astronomical numbers of tiny gametes.

2) Investment. The disparity in gamete size sets the stage for sex differences in investment throughout the entire reproductive cycle. The ovum typically provides half the genetic material of the offspring but also the machinery and nutrients needed to nourish the developing embryos.

Take a chicken’s egg (that is, the ovum). It contains all of the nourishing yolk on which a developing embryo would rely, and these resources are available because of the maternal investment. In stark contrast, in the typical animal species the sperm rarely provides anything else but half of the genetic material.

This initial asymmetry in effort is frequently extended to subsequent stages of offspring development. Think of pregnancy and lactation in a typical female mammal – again, heavily maternally-biased investments.

3) Competition. Sexual asymmetry in gamete numbers and size means eggs are the limiting resource for fertilisations. The large ratio of male to female sex cells means that numerous sperm must therefore compete for access to the much rarer ova.

The consequences of this are far-reaching, in particular because polyandry (females mating with multiple males within the same reproductive cycle) is quite common in nature. That means the competition is not so much among the sperm within an ejaculate but among the sperm of ejaculates of several males.

4) Potential reproductive rates. Summing up all of the above we can deduce that potential reproductive rates for males are typically higher than those of females.

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It is energetically cheaper to produce a male gamete than a female gamete. In general, the sperm stores in males are continually replenished; females, on the contrary, are normally born with a fixed number of ova that cannot be increased, just managed (such as by producing an ovum periodically).

Donkeys produce billions of sperm per ejaculate. A male donkey could therefore theoretically father hundreds or thousands of descendants over a lifetime.

The reproductive success of a female donkey is limited by the number of ova she produces and the higher effort she puts into rearing the offspring.

Enter sexual selection and it all makes sense

The sex that produces small and numerous gametes (that is, the male) will more-often-than-not have higher potential reproductive rates. The primary consequence of anisogamy is therefore that reproductive competition (for mates – i.e. sexual selection) will generally be greater in males.

StoneHorse Studios

Sexual selection shapes traits in males that help them to obtain mates and fertilise as many ova as they can, either because these traits (e.g. the peacock’s train) are preferred by females or because the traits give males an edge that helps them to compete for access to reproductive females and their ova (e.g. deer antlers or faster sperm).

That’s why males across many species exhibit extravagant traits, larger sizes, or behaviours that allow them to increase or secure fertilisations. It also helps to explain why, across species, females are often the sex that tends to be more discerning in their choice of mate.

Of course, none of this is news. The connection between parental investment and sexual selection was formalised years ago, and all that we have said so far encapsulates the basis of what is known to evolutionary biologists as Bateman’s principle.

The answer to our hypothetical survey is simple: gametes are the difference between males and females. But if you take a more holistic point of view, it’s not as straightforward.

Exactly what a male and female looks like, at the end of the day, and how each behaves (their sex roles) depends largely on the ecological theatre in which their evolutionary history has been played out.